Cellular wireless networks are widely known in which base stations (BSs) communicate with terminals (also called user equipments (UEs), or subscriber or mobile stations) within range of the BSs.
The geographical areas covered by base stations are generally referred to as cells, and typically many BSs are provided in appropriate locations so as to form a network or system covering a wide geographical area more or less seamlessly with adjacent and/or overlapping cells. (In this specification, the terms “system” and “network” are used synonymously except where the context requires otherwise). In each cell, the available bandwidth is divided into individual resource allocations for the user equipments which it serves. The terminals are generally mobile and therefore may move among the cells, prompting a need for handovers between the base stations of adjacent and/or overlapping cells.
Handover in wireless systems is conventionally based on downlink (DL) signal quality: when the DL signal quality as measured by the mobile terminal using reference signals transmitted by the base stations drops below a certain threshold, the terminal is handed over to one of the suitable neighbours. Normally, such a handover is effective both for the downlink and the uplink.
One type of cellular wireless communication network is based upon the set of standards referred to as Long-Term Evolution (LTE). In LTE, the measurement most commonly used for handover is the Reference Signal Received Quality (RSRQ), which is an indicator of the wanted signal quality which takes into account the interference levels, or the Reference Signal Received Power (RSRP). The terminal (referred to as a UE in LTE) then uses channel reciprocity to estimate the required power settings for the uplink (UL), by knowing the transmission power of the DL reference signals (which is broadcast by the base station, referred to in LTE as the eNB) and their received power (which the UE measures).
As the UE moves out of the coverage area of its current serving cell, after the RSRQ or RSRP with respect to a neighbour cell exceeds, by a sufficient margin called the “offset”, the RSRQ/RSRP with respect to the serving cell, the terminal transmits this information to the base station of the serving cell, and in one form of handover the serving base station or a higher-level node determines that a handover is required to another “destination” base station. However, various forms of handover are possible in wireless communication networks; for example, the handover decision may be taken by the “destination” base station or even at the terminal.
As an embodiment of the present invention will be described later with respect to LTE, it may be worth briefly outlining some relevant aspects of LTE network topology.
The network topology in LTE is illustrated in FIG. 1. As can be seen, each UE 1 connects over a wireless link via a Uu interface to an enhanced node-B or eNB 11. It should be noted that various types of eNB are possible having differing transmit powers and therefore providing coverage areas (cells) of differing sizes. Multiple eNBs deployed in a given geographical area constitute a wireless network called the E-UTRAN (and henceforth generally referred to simply as “the network”).
Each eNB 11 in turn is connected by a (usually) wired link using an interface called S1 to higher-level or “core network” entities 101, including a Serving Gateway (S-GW), and a Mobility Management Entity (MME) for managing the system and sending control signalling to other nodes, particularly eNBs, in the network. In addition (not shown), a Packet Data Network (PDN) Gateway (P-GW) is present, separately or combined with the S-GW, to exchange data packets with any packet data network including the Internet. Thus, communication is possible between the LTE network and other networks, including other cellular wireless communication networks. It should be noted that in the same geographical area, distinct E-UTRANs (or radio access networks—RANs) may exist using the same, or different, radio access technology (RAT). These networks may be under control of the same operator, or may be coordinated in another way. Thus, inter-RAN and inter-RAT handovers are also possible.
Radio resource management (RRM) is an important aspect in wireless communication networks in order to ensure the efficient use of the available radio resources and to provide mechanisms that enable network to meet radio resource related requirements. In particular, RRM in E-UTRAN provides means to manage (e.g. assign, re-assign and release) radio resources taking into account single and multi-cell aspects. Measurements play a critical role in RRM, especially for mobility and scheduling. Generally network control and configure UEs' measurement and measurement reporting functions. In LTE, the two basic UE measurement quantities are the above mentioned Reference symbol received power (RSRP) and the Reference symbol received quality (RSRQ).
Measurements to be performed by a UE for intra/inter-frequency mobility can be controlled by E-UTRAN, using broadcast or dedicated control. In idle mode, the cell reselection algorithms are controlled by setting of parameters (thresholds and hysteresis values) that define the best cell and/or determine when the UE should select a new cell. Also, E-UTRAN broadcasts parameters that configure the UE measurement and reporting procedures. In connected mode, the mobility of radio connections has to be supported. Handover decisions may be based on UE and eNB measurements. In addition, handover decisions may take other inputs, such as neighbour cell load, traffic distribution, transport and hardware resources and operator defined policies into account.
The following 3GPP standards documents contain useful background and are hereby incorporated by reference:
TS36.300 Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2;
TS36.331 Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification;
TS36.413 Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP); and
TS 36.423 Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2AP).
FIG. 1 shows what is sometimes called a “homogeneous network”; that is, a network of base stations in a planned layout and which have similar transmit power levels, antenna patterns, receiver noise floors and similar backhaul connectivity to the core network. Current wireless cellular networks are typically deployed as homogeneous networks using a macro-centric planned process. The locations of the base stations are carefully decided by network planning, and the base station settings are properly configured to maximise the coverage and control the interference between base stations. However, it is widely assumed that future cellular wireless networks will adopt the structure of the so-called “heterogeneous network”, composed of two or more different kinds of cell.
FIG. 2 depicts a simple heterogeneous network. The large ellipse 10 represents the coverage area or footprint of a Macro cell provided by a base station (Macro BS) 11. The smaller ellipses 20, 22 and 24 represent Micro cells within the coverage area of Macro cell 10, each having a respective base station (Micro BS), one such base station being shown at 21. Here, the Macro cell is a cell providing basic “underlay” coverage in the network of a certain area, and the Micro cells are overlaid over the Macro cell using separate frequency spectrums for capacity boosting purposes particularly within so-called “hot spot zones”. A UE 1 is able to communicate both with Macro BS 11 and Micro BS 21 as indicated by the arrows in the Figure. Thus, for example, the same UE may use both the Macro cell as its “primary” cell (Pcell) and a Micro cell as a “secondary” cell. When a UE starts to use a given cell for its communication, that cell is said to be “activated” for that UE, whether or not the cell is already in use by any other UEs.
The Radio Access Technology (RAT) adopted by the base stations could be any kind, for example, 3G or 4G. Here we assume that a 4G RAT such as 3GPP Long-Term Evolution (LTE) is adopted by each of the cells in the network and use this as an example to illustrate the proposed method. Although only two types of cell, Macro or Micro, are shown in FIG. 2, various levels of cell are under consideration for 4G including so-called Femto and Pico cells. Femto and Pico cells can be overlaid on either Macro or Micro cells as explained below. Also, in LTE each Macro eNB generally is sectorized into N (N>=1) partitions, each of which or any subset of which may constitute a cell. A typical example is for the base station to have three sectors, each of which is configured as a cell with frequency reuse factor being 1. Therefore, references to “cell” therefore include “sector” unless where the context demands otherwise.
A more complex heterogeneous network may consist of Femto, Pico, Micro and Macro base stations. Of these, the operator will have control over Pico, Micro and Macro Base stations. Femto base stations are expected to be installed by users, with backhaul provided by broadband Internet, and consequently activation/deactivation thereof is not under control of the network operator. FIG. 3 shows the operator-controlled cells in part of such a heterogeneous network.
The three biggest cells 10, 12 and 14 represent the Macro cells in the network, while the medium sized cells are Micro cells and the smallest cells are Pico cells. Within each Macro cell, Micro cells exemplified by 26 and 28 provide a first level of additional capacity. It should be noted that Micro cell 28 is at least partly within the coverage area of two Macro cells, 10 and 12. Within the Micro cells, in turn, there are Pico cells illustrated by the small circles and exemplified by 30 and 32. Pico cell 30 is an example of a Pico cell which is within the coverage area of a Micro cell 26, as well as within the coverage area of Macro cell 10. Pico cell 32 is an example of a Pico cell which is within the coverage area of a Macro cell only.
The network is designed such that the Macro cells provide blanket coverage while the smaller Micro and Pico cells are providing additional capacity.
Thus, in addition to the Macro cell to Micro cell relationship shown in FIG. 2, where the Micro cells provide additional capacity to the basic coverage provided by the Macro cells, it is possible to define a Pico cell to Micro cell relationship, where the Pico cells provide additional capacity to that of the Micro cells which are already serving as capacity boosters, as well as a Pico cell to Macro cell relationship, where the Pico cells provide additional capacity to the basic coverage provided by the Macro cells. The above mentioned Femto cells may provide a further layer of coverage. For present purposes, all of the Micro, Pico and/or Femto cells as may be present in a heterogeneous network can be regarded as “Small Cells”.
The principal scenario of interest in this invention is an LTE heterogeneous network (HetNet) where UEs operate within the coverage of at least two cells: a Macro Cell and a Small Cell (low power node, e.g. a Pico Cell, a relay or a Femto cell). The Small Cell eNodeB (SCeNB) and the Macro Cell eNodeB (MeNB) are able to exchange information over the X2 interface or S1 interface, through which several procedures/functions can be executed/coordinated between the MeNB(s) and SCeNBs. For example, UEs can be handed over between the neighbouring MeNB(s) and SCeNB(s); or inter-site/eNB carrier aggregation or co-operative multipoint transmission (COMP) can be operated among the neighbouring MeNB(s) and SCeNB(s); or dual connectivity of the UEs can be maintained where UEs have multiple connections with the MeNB(s) and SCeNBs.